Spirometry is the art of determining the health and capacity of the lungs. Original instruments comprised a mouthpiece, blowtube, water cylinder, ballast and chart recorder. The patient blew into the mouthpiece, thereby lifting the ballast a measurable height for a measurable time. Time versus displacement graphs could be combined with the patient's age, weight and medical history to help diagnose lung diseases.
In the past few years the use of computers for spirometry measurement has become commonplace. As a consequence the American Thoracic Society (ATS) publishes and updates a Standardization of Spirometry for spirometry systems including the disposable mouthpiece.
Modern health care facilities perform countless spirometry tests annually. Spirometers can be the cause of spreading bacteria and viruses resulting in the spread of serious diseases including TB and HIV. Cleansing reusable mouthpieces and instrument parts is a costly and risky procedure. This is especially true considering the control procedures necessary for monitoring compliance of numerous test personnel working under time pressure.
The result of the undesirability of cleansing reusable mouthpieces and instrument parts has led to the development of several disposable mouthpiece designs. Before discussing the development of disposable mouthpieces we must first review some testing basics.
There are three basic tests performed by spirometers. They are first a forced vital capacity test, second a diffusion test, and third a lung volume test. Spirometers used in the office of a physician generally perform only the vital capacity test while the instruments used by a hospital or pulmonary physician are usually capable of performing all three tests. These instruments which perform all three tests are sometimes called total lung analyzers, but they are actually spirometers.
The forced vital capacity test simply measures time versus flow. Low filter resistance is required to assure accurate flow readings. The diffusion test measures the efficiency of oxygen transfer from the alveoli to the blood. This test uses minute amounts of carbon monoxide breathed into the lungs through a closed loop spirometry system. This test requires low dead space in the filter apparatus to assure accurate readings of minute amounts of carbon monoxide. Finally the lung volume test uses a small amount of helium in a closed loop system to measure the lungs' residual air after expiration. This test also requires a leak proof seal in the filter apparatus to prevent leakage of helium.
Thus these three tests require different characteristics from the filter apparatus. These ideal characteristics are:
1) Low resistance PA1 2) High filtration efficiency of bacteria PA1 3) Low dead space PA1 4) Leak proof seal PA1 5) Low cost PA1 6) Universal mounting capabilites on dozens of spirometers PA1 7) Reduced volumetric disposal of plastics PA1 1) low resistance at 0.9 cm H.sub.2 O/liter/second at 12 liters per second flow. PA1 2) 99.99% and 99.98% efficient in stopping airborn bacteria and viruses respectively. PA1 3) Low dead space ranging from 90 to 115 cc's. PA1 4) Leak proof design. PA1 5) Low cost at $1.65 each. PA1 6) Universal mounting adapters. PA1 7) Only the mouthpiece is disposed, thus cutting the waste plastic in half, or for a reusable mouthpiece embodiment, only the filter pad is disposed.
Further economic and environmental characteristics of the ideal filter are:
It should be noted that low resistance is most easily obtained by having a large dead space and a wide area for gas flow through the filter. Also a minimal filtration efficiency offers a low resistance.
Two main disposable filter systems comprise the prior art. First the Pall Barrier Filter.TM. uses a baffle type barrier which is 99.9% effective in filtering particulate matter including spit. But the barrier is noneffective on airborn bacteria on the order of 0.2 microns. The Pall filter has a very low resistance around 0.4 cm H.sub.2 O/liter/second at 12 liters per second flow. The ATS standard for the total spirometry instrument is less than 1.5 cm H.sub.2 O/liter/second at 12 liters per second flow. The internal dead space of the Pall filter is very low at 40 cc. The cost is quite high at $5 per unit. In summary the Pall filter fails to block airborn bacteria (ATS specs do not specify a standard) and is expensive.
The second known disposable filter is the Marquest.TM. filter. It uses a 3M.RTM. gauze filter named Filtrete.RTM. at a thickness of 200 gm/sq.m. It is 99.99% efficient in filtering airborn bacteria and 99.98% efficient for viruses. Thus, the Marquest filter efficiently filters bacteria and viruses. However, in order to obtain a resistance of about 0.9 cm H.sub.2 O/filter/second at 12 liters per second flow, Marquest creates a large 31/2" diameter orifice thus a large surface area for the Filtrete.RTM., thereby creating about 150 cc's of dead space. ATS calls for 150 cc's of dead space for the total spirometer. Thus, the Marquest filter is not suitable for either the diffusion test or the lung volume test. The price, however, at $3.50 is quite an advantage over the Pall filter.
Another embodiment of the Filtrete.RTM. filter uses too thin a gauge of Filtrete.RTM. spread across a 2" diameter orifice. This approach creates an acceptably low dead space but an unacceptably high resistance. The filtration efficiency is also dramatically reduced. The cost is very low at $1.00 each. Thus, this embodiment does not satisfy the market demands.
The present invention uses interchangeable housing components in combination with 31/2" diameter Filtrete.RTM. discs to meet all seven market demands: